Radiation therapy, also called radiotherapy, is an important tool in medicine, particularly in therapeutic cancer treatment, where ionizing radiation is used to control malignant cells found, for example, in malignant tumors. The doses of radiation used in radiation therapy are significantly higher than those used in radiology i.e. medical imaging and diagnosis. Usually the radiation is applied using an external radiation source, an accelerator, which generates a beam of high-energy photons. The photons will predominantly, through Compton scattering, produce secondary electrons. Most high energetic electrons will continue in almost the same direction as the scattered photon while low energetic electrons will move randomly in other directions. The ionizing radiation may alternatively comprise other particles such as electrons, protons, etc. Normally the accelerator is mounted on top of a gantry, and the radiation beam is directed towards the patient under treatment who is positioned in the middle of the gantry, as illustrated in FIG. 1. Detectors for imaging and dosimetry are placed in the bottom part of the gantry in order to detect radiation that has passed through the patient.
A radiation beam damages cells by ionization of the atoms in the DNA molecules and the objective of radiation therapy is to attain a high dose to the tumor, while the dose to the surrounding tissue is minimized in order to avoid harmful side effects. The harmful effect on surrounding normal tissue and the skin can be reduced by applying the treatment during a plurality of sessions. However, it is more efficient and safer to reduce the dose applied to normal tissue during the treatment, e.g. by applying the radiation in small doses transmitted from a plurality of angles so that each dose takes a different path through normal tissue. Moreover, the treatment volume may be adapted so that it conforms to the shape of the tumor by shaping the beam, whereby the relative toxicity of radiation to the surrounding normal tissues is reduced, allowing a higher dose of radiation to be delivered to the tumor. In intensity-modulated radiation therapy (IMRT), which has been one of the most important advances in conformal radiotherapy, the profile of the beam is shaped and the intensity of the beam over the cross-sectional area of the beam is varied, conforming the treatment volume further.
Conventional radiotherapy treatment requires thorough dose planning, access to calibrated radiation sources and methods for dosimetry. The first step in the planning is to anatomically localize the tumor and to identify the absorption properties of the surrounding tissue. One of the most useful tools in this is computed tomography (CT). Thereafter the kind of radiation beam, i.e. photon, electron or proton beam, the shape, directions and energy thereof, distance to radiation source target dose, etc. are selected. For accurate planning and treatment the radiation sources needs to be calibrated, which conventionally is accomplished using ionization chambers or other detectors placed in so-called phantoms prior to the treatment. During treatment dosimetry detectors are used in specific locations only for monitoring purposes in order to protect the patient from overdose. Photographic imaging would be much more convenient, but is however not feasible due to reasons explained in the following. A final plan for the radiation treatment is established by simulation and the treatment is performed essentially based on this plan. Treatment verification today involves comparison of an image acquired during a treatment fraction with a reference image that is generated prior to the initiation of the treatment. The alignment of the beam is normally determined from an X-ray image (kEV-beam) of the patient acquired immediately before the treatment starts.
Without real-time field monitoring during the therapy process there is a risk that errors during the planning examination or changes in conditions after the X-ray examination or during treatment, e.g. due to inadequate patient immobilization, may lead to reduced efficiency of the therapy and/or damage to surrounding natural tissue. Especially with IMRT, verification of the actual dose intensity and position is important and a fast feedback between the accelerator providing the beam and a detector monitoring the field is invaluable. A correction of the patient set-up using the information from a beam monitor increases the probability of a complication-free tumor cure in 10% of cases (J. Löf, B. K. Lind and A. Brahme, Phys. Med. Biol, 43, p 1605-1628, 1998).
In radiology there are several methods for imaging: X-ray imaging, computed tomography (CT), and positron emission tomography (PET) to mention just a few. These and other methods are however not adapted to the high energy beams used in radiation therapy.
Portal imaging provides an image of the irradiated region using the therapeutic beam. The conventional imaging media is photographic film, but electronic portal imaging devices (EPID) are becoming common. Images with fairly high quality can be produced using portal imaging films, but one inherent drawback is that the film has to be processed and normally also digitized before it is possible to obtain any feedback on the radiation therapy process. At best EPIDs enable real-time images and at worst they at least remove the time-consuming step of transferring the films to a (digitized) format wherefrom analysis can be made. The geometrical information provided by portal imaging is mainly used to ensure that the target region for the treatment (e.g. a tumor) is in the correct position during the treatment.
Several electronic portal imaging systems are already commercially available and the dominating technologies are either camera-based or liquid ion chamber arrays. Lately amorphous silicon based electronic portal imaging devices have received increasing attention. The image quality for currently available instruments is usually rather poor.
In nearly all commercial detectors for medical X-ray imaging the principle of charge integration is used. In charge integration detectors, such as charge-coupled devices (CCD) and flat panel detectors (FPD), the noise is added to the total deposited energy and the incident photons are given a weight proportional to the amount of charge deposited, i.e. proportional to the photon energy. Devices that utilize a photon counting principle are currently appearing. In photon counting detectors all photons that deposit a charge larger than a predefined threshold give an equal counter response. No information about the energy of the photon is preserved.
Imaging devices generally comprise an array of detector cells. Incoming radiation generates a charge in the detector cell that is read out either in sequence from all of the cells to a common output circuit or each detector cell is connected to a separately addressable circuit, which enables charge detection from individual cells. WO95/33332 discloses an imaging device with an array of individually addressable imaging cells wherein successive photon hits can be accumulated and read out from each cell after a certain time period. These devices are not able to distinguish between different photon energies. The international application WO98/16853 “Imaging Device for Imaging Radiation” discloses another imaging device with a similar arrangement of an array of image cells and individually addressable circuits connected to each image cell, wherein each circuit comprises an additional counting circuit. Thereby the number of radiation hits can be read out together with the accumulated charge.
There is an increasing demand for portal imaging devices able to determine the patient dose in addition to the geometrical information. The requirements for dosimetry and imaging are very different, hence a good image can not readily be extracted from dosimetry data and, similarly, dose cannot be accurately estimated from an image.
In general, geometric verification requires the portal image to be registered with a reference image and dosimetric verification requires the portal imager to be calibrated for dose. Therefore real-time imaging combined with real-time adjustment of the therapeutic beams is not easily performed using conventional technology.
The contrast in an image can be enhanced by single photon counting, wherein the signal generated in the detector cell is compared with a predefined threshold level enabling only hits within a predetermined energy range to be counted. In this way so-called soft photons, i.e. low energy photons originating from scattering, that dilute the image can be excluded. Single photon counting is however not advantageous for measuring dose, since the important energy information of the photon is lost in the discrimination process.